1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, or facsimile device for forming an image using a two-component developer constituted by a toner and a magnetic carrier, and an image density control method.
2. Description of the Related Art
A two-component development system in which a two-component developer (to be referred to hereafter simply as “developer”) constituted by a toner and a magnetic carrier is held on a developer carrier, the developer is used to form a magnetic brush by means of a magnetic pole provided in the interior of the developer carrier, and development is performed by sliding the magnetic brush over a latent image formed on a latent image carrier is known widely in the related art. The two-component development system is used widely due to the ease with which color images can be formed thereby. In the two-component development system, background staining and a reduction in detail resolution may occur on a formed image when the toner concentration, i.e. the ratio (for example, the weight ratio) between the toner and the magnetic carrier in the developer is too high. When the toner concentration is too low, on the other hand, the density of the solid image portion may decrease, and the carrier may adhere to the latent image carrier. It is therefore important to perform toner concentration control to detect the toner concentration of the developer in a development device and control a toner replenishment operation so that the toner concentration of the developer is always within an appropriate range.
Furthermore, it is generally important to perform image formation in an image forming apparatus such that a constant image density is obtained at all times. The image density is principally determined according to the development ability of the development device. The development ability is determined according to the amount of toner that can be adhered to a latent image during development, and varies according to the toner concentration, development conditions such as the development potential, which is the potential difference between the latent image on the surface of the latent image carrier and the development carrier surface to which a developing bias is applied, and the charge of the toner that contributes to development. The incline (development γ) of a relational expression indicating the toner adhesion amount relative to the development potential is widely used as an index of the development ability. Since the image density is determined according to the development ability of the development device, the image density cannot be fixed simply by performing toner concentration control such that the toner concentration is always within an appropriate range, as described above. Moreover, although development conditions such as the development potential can be fixed comparatively easily, it is difficult to fix the charge of the toner that contributes to development. Hence, even if the development conditions are fixed and toner concentration control is performed to fix the toner concentration, the development ability cannot be fixed, and therefore a constant image density cannot be obtained.
More specifically, when an image with a low image area ratio is output, for example, the amount of toner consumed during development is comparatively low, and therefore the amount of replenishment toner required to maintain a desired toner concentration is small. Accordingly, the amount of toner that remains in the development device for a comparatively long time is large. The toner that remains in the development device for a comparatively long time is agitated for a long time, and therefore the toner in the development device is more likely to be excessively charged. Hence, the development ability is comparatively low. In contrast, when an image with a high image area ratio is output, the amount of new replenishment toner that is not sufficiently charged is large, and therefore the proportion of toner that is not charged to the desired charge within the toner that contributes to development is large. As a result, the development ability is comparatively high. In recent years, there has been a trend toward reducing the amount of developer in the development device as much as possible in response to demands for reductions in the size of the development device. As a result, the toner that contributes to development during image formation after outputting an image with a high image area ratio has a larger proportion of toner that is not charged to the desired charge. Hence, the development ability during image formation after outputting an image with a high image area tends to become comparatively high.
Depending on the structure, the development ability may become higher when an image with a low image area ratio is output than when an image with a high image area ratio is output. For example, when toner having an adhered additive is used and a development device in which the toner is subjected to a high level of stress is employed, the additive may become buried in or separate from the toner surface of the toner that has existed in the development device for a comparatively long time as a result of long-term agitation. When a large amount of such toner exists, the fluidity of the developer deteriorates and the charging ability of the toner decreases, and as a result, the toner that contributes to development cannot be charged to the desired charge. Hence, when an image having a low image area ratio is output, the proportion of toner that is not charged to the desired charge within the toner that contributes to development increases, and as a result, the development ability becomes comparatively high. In contrast, when an image with a high image area ratio is output, the replenishment toner amount is large, and therefore the amount of toner that has existed in the development device for a comparatively long time is small. Therefore, the fluidity of the developer is sufficiently favorable, and the amount of toner having a sufficient charging ability is large. Accordingly, the toner that contributes to development can be charged to the desired charge, and therefore the development ability is comparatively low.
As described above, the proportion of new toner in the development device following toner replenishment differs according to whether toner replenishment is performed after outputting an image having a low image area ratio or after outputting an image having a high image area ratio, and this difference leads to variation in the development ability. Hence, even when the development conditions are fixed and toner concentration control is performed to fix the toner concentration, the development ability cannot be fixed, and therefore a constant image density cannot be obtained.
An image forming apparatus described in Japanese Unexamined Patent Application Publication S57-136667 and Japanese Unexamined Patent Application Publication H2-34877 may be cited as an example of an apparatus capable of suppressing this problem. In this image forming apparatus, toner concentration detecting means are provided for detecting and outputting the toner concentration of a two-component developer in a development device. The output value of the toner concentration detecting means is compared to a toner concentration control reference value, and a toner replenishment device is controlled on the basis of the comparison result such that the toner concentration of the two-component developer in the development device reaches a desired toner concentration. Then, by detecting the density of a reference toner pattern formed in a non-image portion, the image density during formation of the reference toner pattern is learned, and on the basis of the detection result, the toner concentration control reference value is corrected. According to this method, image formation can be performed at a desired image density for a certain period of time following correction of the toner concentration control reference value. Hence, by forming the reference toner pattern and correcting the toner concentration control reference value in accordance with the detection result periodically, a constant image density can be obtained.
However, in the image forming apparatus described in these publications, the reference toner pattern must be formed every time the toner concentration control reference value is corrected, leading to an increase in the amount of toner consumed for a purpose other than image formation.
To solve this problem, an image forming apparatus described in Japanese Patent Application No. 2005-327647 comprises information detecting means for detecting information for learning a toner replacement amount in a development device over a predetermined time period, for example the image area ratio of an output image. From the detection result of the information detecting means, the ratio of new toner or old toner in the development device is learned, and thus the development ability of the development device is learned. Furthermore, a toner concentration control reference value is corrected by toner concentration control reference value correcting means on the basis of the detection result of the information detecting means, and by adjusting the toner concentration in the development device, a constant image density is obtained. In this image forming apparatus, the information regarding the toner replacement amount, which is used to correct the toner concentration control reference value, can be detected without consuming toner to detect the image area ratio of an output image or the like, and therefore increases in the amount of toner consumed for a purpose other than image formation can be suppressed.
However, this image forming apparatus is incapable of responding to variation in the development ability of the development device due to factors other than the toner replacement amount in the development device over a predetermined time period, for example environmental variation, the standing time, and so on. Therefore, the image density cannot be controlled appropriately by the inventions proposed in the related art.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Application Publication 2005-331720.